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Abstract

The conventional way of biasing a condenser microphone is using a resistor to the supply voltage. This scheme is simple, but has several disadvantages such as poor supply line noise rejection, loss of microphone sensitivity due to the voltage divider formed by the microphone's internal impedance at the signal frequency and the bias resistor, and larger variation in microphone sensitivity due to variation in bias voltage caused by variation in the required bias current between microphones.

Copyright

Motorola Inc. October 1988

Country

United States

Language

English (United States)

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MOlYlROLA Technical Developments Volume 0 October 1988

CONSTANT VOLTAGE APPROACH TO CONDENSER MICROPHONE BIASING

by Ole Hammer and Bob McDermott

The conventional way of biasing a condenser microphone is using a resistor to the supply voltage. This scheme is simple, but has several disadvantages such as poor supply line noise rejection, loss of microphone sensitivity due to the voltage divider formed by the microphone's internal impedance at the signal frequency and the bias resistor, and larger variation in microphone sensitivity due to variation in bias voltage caused by variation in the required bias current between microphones.

A different approach that solves these problems are shown in the figure below. The non-inverting input of the amplifier is set to the desired bias voltage. Depending on the application, the bias voltage will typically be in the range of 1.5 volts to 10 volts. The bias current is provided from the amplifier output via the feedback resistor, R. The gain of the amplifier is controlled by the ratio between the feedback resistor, R, and the microphone internal impedance at the signal frequency. The feedback resistor, R, must be chosen to provide adequate bias current under all circumstances, while maintaining the amplifier within its dynamic range. The capacitor, C, provides AC - coupling to the next stage in the audio path. By taking this approach, the supply line noise rejection is improved to be that of the amplifier, no signal is los...